The invention relates to cleaning agents for ultrafiltration membranes in ultrafiltration units of electro-dipcoating plants. The invention relates also to a process for cleaning ultrafiltration membranes using the cleaning agents.
Electro-dipcoating (EDC) is widely used in industrial lacquering both as anodic dipcoating (ADC) and, especially, as cathodic dipcoating (CDC). In particular, it is used, for example, as CDC in the automotive industry for applying the anticorrosive primer coating layer to motor vehicle bodies. The EDC baths are coupled with an ultrafiltration unit having one or more ultrafiltration modules, in which unit the electro-dipcoating lacquer (EDL) is subjected to membrane pressure filtration at, for example, from 1 to 5 bar (initial pressure at the ultrafiltration module). During the ultrafiltration, the EDC coating composition is separated at a semi-permeable membrane into a concentrate, which is fed back into the EDC bath, and an EDL ultrafiltrate (permeate). The EDL ultrafiltrate has only a low solids content of, for example, from greater than 0 to less than 0.5 wt. %. It consists mainly of water, and it contains to a subordinate degree constituents of the electro-dipcoating lacquer that pass through the membrane, such as neutralising agents, dissolved salts, organic solvents and low molecular weight binder constituents. The EDL ultrafiltration has two functions. On the one hand, the EDL ultrafiltrate that is obtained is used to rinse non-adhering EDC lacquer from the EDL-coated substrates after they emerge from the EDL coating bath. After rinsing, the EDL-coated substrates are conveyed to the stoving furnace of the EDC plant. The EDL rinsed off with the EDL ultrafiltrate is fed back to the EDC bath again. The second function of the EDL ultrafiltration is that of keeping the composition of the EDC bath constant within a certain tolerance, which is achieved by the discarding of a portion of the EDL ultrafiltrate that is not used as rinsing agent. In that manner, it is possible to prevent disruptive constituents that pass through the membrane from building up in the EDC bath.
The separating membranes within the ultrafiltration modules of EDL ultrafiltration units require cleaning at intervals and, generally, also conditioning (impregnation) before being used for their intended purpose. U.S. Pat. No. 4,136,025, for example, describes a process for cleaning membranes within CDL ultrafiltration units using an aqueous solution of an acid containing a water-soluble organic solvent. The example mentioned in U.S. Pat. No. 4,136,025 gives the following as the composition of the aqueous cleaner solution:                7.5 wt. % butyl glycol,        5.7 wt. % 88% aqueous lactic acid, and        86.8 wt. % deionised water.        
The cleaning action of such cleaning agents of the prior art consisting essentially of neutralising agent, water and organic solvent is in need of improvement. For example, it has been found that, following membrane cleaning using cleaning agents having such a composition, the performance of the EDL ultrafiltration units declines ever more rapidly from cleaning interval to cleaning interval in the sense of an exponential function, that is to say the cleaning intervals become shorter from one cleaning operation to the next until cleaning is no longer worthwhile and the membrane or the ultrafiltration modules containing the membrane have to be replaced. In addition, the performance of the EDL ultrafiltration units becomes poorer and poorer from cleaning interval to cleaning interval as compared with the performance following each preceding cleaning operation. Such disadvantages of the use of cleaning agents of the prior art are particularly pronounced in connection with the cleaning of modem EDL ultrafiltration modules of compact construction.
The object of the invention is to provide cleaning agents that are more effective as compared with the prior art, and processes for cleaning EDL ultrafiltration membranes.
The object can be achieved if aqueous cleaning agents containing EDL binders overneutralised with neutralising agent are used for the cleaning of EDL ultrafiltration membranes, for example ultrafiltration membranes contained in EDL ultrafiltration modules of EDL ultrafiltration units.